The present invention relates to a process for recovering fluoropolymers from fluorine-containing ion exchange membranes used for e.g. electrolysis of sodium chloride.
Fluorine-containing ion exchange membranes are widely used as diaphragms for electrolytic cells for the production of sodium hydroxide by electrolysis of sodium chloride. As such a fluorine-containing ion exchange membrane, a laminated ion exchange membrane having from 2 to 4 laminated films of a fluoropolymer having carboxylic acid groups and a fluoropolymer having sulfonic acid groups, or one having such a laminated ion exchange membrane reinforced by a woven fabric made of polytetrafluoroethylene (hereinafter referred to as PTFE).
Further, for the purpose of preventing deposition, on the membrane surface, of a gas generated during electrolysis and reducing the voltage for electrolysis, one having inorganic particles of e.g. silicon carbide or zirconium oxide coated on the surface of a laminated ion exchange membrane, is used. On the other hand, on the surface of the ion exchange membrane, precipitates composed mainly of iron oxide or a hydrate thereof (hereinafter referred to as surface precipitates) are likely to deposit during the electrolysis.
Heretofore, when the performance of ion exchange membranes used for the electrolysis decreased, it was common that they were dismounted from the electrolytic cell and used or disposed for e.g. land filling as wastes. However, in recent years, it is desired to recover and reuse fluoropolymers constituting the ion exchange membranes as materials for ion exchange membranes or membranes for fuel cells, from the viewpoint of avoiding the influence over the environment.
In order to recover and reuse a fluoropolymer having carboxylic acid groups and a fluoropolymer having sulfonic acid groups from ion exchange membranes, it is necessary not only to separate the two but also to remove the above-mentioned woven fabric and inorganic particles from the ion exchange membranes.
The following methods have therefore been known as a method for recovering fluoropolymers from fluorine-containing ion exchange membranes. A method wherein laminated fluorine-containing ion exchange membranes comprising two or more fluoropolymer layers having e.g. carboxylic acid groups and/or sulfonic acid groups, are converted to acid-forms or alkali metal salt-forms and then immersed in a water-soluble organic solvent to elute the respective fluoropolymers, and the respective fluoropolymers are recovered from the eluted solution (JP-B-3-14860), or a method wherein laminated ion exchange membranes comprising a fluoropolymer having carboxylic acid groups and a fluoropolymer having sulfonic acid groups, are immersed in a fluoroalcohol to elute the fluoropolymer having sulfonic acid groups (JP-A-2000-86809).
However, in these methods, it is difficult to remove inorganic particles, whereby there will be a problem that the obtainable fluoropolymers are of low purity.
It is an object of the present invention to provide a process for efficiently recovering a fluoropolymer having carboxylic acid groups and a fluoropolymer having sulfonic acid groups, respectively, in high purity, by removing inorganic particles from an ion exchange membrane comprising the fluoropolymer having carboxylic, acid groups, the fluoropolymer having sulfonic acid groups and the inorganic particles deposited on the surface.
The present invention provides a process for recovering fluoropolymers, which comprises contacting an ion exchange membrane comprising a fluoropolymer having carboxylic acid groups (hereinafter referred to as a C-polymer) and a fluoropolymer having sulfonic acid groups (hereinafter referred to as a S-polymer) and having inorganic particles deposited on the surface, with a solvent which is a good solvent for the C-polymer and a good solvent for the S-polymer, to separate a solution having the C-polymer and the S-polymer dissolved therein, and the inorganic particles, then esterifying the C-polymer in the solution to form precipitate of an ester of the C-polymer, and fractionating and recovering the precipitate and a solution having the S-polymer dissolved therein by solid-liquid separation.
Here, the inorganic particles include not only particles of e.g. silicon carbide or zirconium oxide deposited on the surface of the ion exchange membrane to prevent attachment of gas, but also surface precipitates, etc. deposited during the electrolysis. Further, in this specification, the C-polymer includes not only one in the form of an acid-form but also one wherein a part or whole of the fluoropolymer having carboxylic acid groups is in the form of a salt-form. Likewise, the S-polymer includes not only one in the form of an acid-form but also one in which a part or whole of the fluoropolymer having sulfonic acid groups is in the form of a salt-form.
According to the present invention, inorganic particles on the surface of an ion exchange membrane are preliminarily removed, and then the C-polymer and the S-polymer are recovered, whereby the C-polymer and the S-polymer can be obtained in high purity. Further, the C-polymer in the solution having the C-polymer and the S-polymer dissolved therein, is esterified to form precipitate of an ester of the C-polymer, whereby the C-polymer and the S-polymer can easily be separated and recovered by solid-liquid separation.
Further, in a second aspect, the present invention provides a process for recovering fluoropolymers, which comprises treating an ion exchange membrane comprising a C-polymer and a S-polymer and having inorganic particles deposited on the surface, with a solvent to let it swell and to remove the inorganic particles, then contacting it with a solvent which is a good solvent for the C-polymer and a good solvent for the S-polymer to obtain a solution having the C-polymer and the S-polymer dissolved therein, esterifying the C-polymer in the solution to form precipitate of an ester of the C-polymer, and fractionating and recovering the precipitate and a solution having the S-polymer dissolved therein by solid-liquid separation.
According to this process, the ion exchange membrane is swelled by a solvent, whereby inorganic particles can easily be separated from the ion exchange membrane, and the C-polymer and the S-polymer can be obtained in high purity.
Further, in this specification, the solvent which is a good solvent for the C-polymer and a good solvent for the S-polymer, will be hereinafter referred to simply as a good solvent. Further, the solvent to let the ion exchange membrane swell thereby to remove inorganic particles, will be hereinafter referred to as a swelling solvent.
The following compounds may be mentioned as preferred good solvents in the present invention. An alkyl alcohol such as methanol, ethanol, n-propanol or i-propanol, a solution which is a mixed solution comprising the above alkyl alcohol and water, wherein the content of water is at most 30 mass%, an amide such as N,N-dimethylformamide, N,N-dimethylacetoamide or N-methylpyrrolidone, a ketone such as acetone or 2-butanone, an ether such as diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, 1,2-dimethoxyethane, tetrahydrofuran or 1,4-dioxane, a nitrile compound such as acetonitrile, a sulfur-containing compound such as dimethylsulfoxide or sulfolane, and a fluorocompound such as 1,1-dichloro-2,2,3,3,3-pentafluoropropane, 1,3-dichloro-1,1,2,2,3-pentafluoropropane, 1,1-dichloro-2,2,2-trifluoroethane, 1,1,1,2,3,3-hexafluoropropane, 1,1,1,2,3,4,4,5,5,5-decafluoropentane, perfluoro(n-butylmethyl)ether, 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,3,3,3-pentafluoro-1-propanol, 2,2,2-trifluoroethanol or 2,2,3,3-tetrafluoropropanol.
Especially, when the good solvent is an alkyl alcohol such as methanol, ethanol, n-propanol or i-propanol, or a mixed solution comprising such an alkyl alcohol and water, such a good solvent can be used as it is for the esterification reaction of the C-polymer, such being preferred. Particularly preferred is methanol or an aqueous methanol solution wherein the content of water is at most 30 mass%.
Further, in the ion exchange membrane after being used for electrolysis of sodium chloride, the fluoropolymers are mostly in the form of a sodium salt whether they have carboxylic acid groups or sulfonic acid groups. Accordingly, with a view to increasing the solubility in the good solvent, it is preferred to treat the C-polymer and the S-polymer with an acid to convert them from the salt-forms to the acid-forms before or at the time of contacting the ion exchange membrane with the good solvent.
As the acid to be used for this purpose, hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid or acetic acid is, for example, preferred. Particularly preferred is hydrochloric acid. Further, in order to neutralize most of the salt, it is preferred that the amount of the acid is at least 1 equivalent to the total of the salt type ion exchange groups in the C-polymer and the salt type ion exchange groups in the S-polymer. On the other hand, the amount of the acid is preferably at most 10 equivalents, to the total of the above salt type ion exchange groups.
Further, the preferred concentration of the acid to be used, is from 0.5 to 20 mass%, although it may vary depending upon the type of the acid. This treatment of the ion exchange membrane with the acid is preferably carried out at the time of separating the inorganic particles.
The temperature at the time of contacting the ion exchange membrane with the good solvent, varies depending upon the solvent, the type of the acid to be added and their mixing ratio, but it is usually preferably from 0 to 100xc2x0 C., particularly preferably from 15 to 50xc2x0 C. At that time, the pressure may be normal pressure, but with a view to increasing the solubility, dissolution may be carried out under elevated pressure.
Further, in the second process of the present invention, the ion exchange membrane is swelled by the swelling solvent to separate and remove the inorganic particles, before contacting the ion exchange membrane with the good solvent. The following methods may be mentioned as preferred methods for separating the inorganic particles.
1) The ion exchange membrane is fixed. Then, the swelling solvent is continuously supplied and contacted with the ion exchange membrane by a method such as refluxing under heating or circulation by pumping. As the ion exchange membrane swells, the inorganic particles will be separated from the ion exchange membrane and dispersed in the swelling solvent. For the purpose of accelerating the separation of the inorganic particles, the ion exchange membrane may be shaked when the solvent is contacted with the ion exchange membrane.
2) The ion exchange membrane is cut into a proper size, preferably from 1 to 30 square cm, and put into the swelling solvent, followed by stirring. The stirring time is preferably at least 5 hours, particularly preferably at least 20 hours. At that time, it is preferred to heat the solvent for the purpose of accelerating the swelling. Then, the inorganic particles will be removed by a method wherein sieving is carried out so that the ion exchange membrane will remain on the sieve and thus will be separated from the solvent having the inorganic particles dispersed therein, or a method wherein the inorganic particles in the solvent will be precipitated and removed from a mixture comprising the ion exchange membrane and the solvent having the inorganic particles dispersed therein, and then the ion exchange membrane will be withdrawn.
The swelling ratio when the ion exchange membrane is permitted to swell is preferably at least 1.2, particularly preferably from 1.3 to 3.0. Here, the swelling ratio is the ratio of the mass of the ion exchange membrane after swelling to the mass of the ion exchange membrane before swelling. When the swelling ratio is at least 1.2, the inorganic particles can sufficiently be separated. Further, when the swelling ratio is at most 3.0, handling of the ion exchange membrane is easy, such being preferred.
The swelling solvent to let the ion exchange membrane swell, is preferably a mixed solvent comprising a water-soluble organic solvent and water, whereby the solubility of the fluoropolymers is low. As a preferred water-soluble organic solvent, methanol, ethanol, n-propanol, i-propanol, dioxane, acetone, sulfolane, an ethylene glycol or a propylene glycol may be mentioned. Among them, from the viewpoint of efficiency in recovery and reuse of the solvent, methanol, ethanol, isopropyl alcohol or acetone is particularly preferred, and especially from the viewpoint of the handling efficiency, methanol or ethanol is preferred.
The content of water in the mixed solvent comprising a water-soluble organic solvent and water, varies depending upon the type of the water-soluble organic solvent, the type and amount of the acid to be added and the temperature, but is preferably at least 50 mass%, particularly preferably from 80 to 95 mass%. Further, an especially preferred mixed solvent of an organic solvent and water, is an aqueous ethanol solution, wherein the content of water is from 60 to 98 mass%.
Further, the temperature to let the ion exchange membrane swell, varies depending upon the solvent, the type of the acid to be added and their blend ratio, but it is preferably from 0 to 100xc2x0 C., particularly preferably from 15 to 50xc2x0 C. If the temperature is too high, the amounts of the C-polymer and the S-polymer dissolved in the solvent increase, whereby the recovery rates of the C-polymer and the S-polymer will decrease. The pressure to let the ion exchange membrane swell, may be normal pressure or elevated pressure.
Further, the method for esterifying only the C-polymer in the solution having the C-polymer and the S-polymer dissolved therein, is preferably a method wherein the above solution is heated and reacted together with an alcohol. In such a reaction, it is preferred to carry out the reaction by adding e.g. an acid such as hydrochloric acid, sulfuric acid or phosphoric acid, or thionyl chloride. In the above method, the alcohol is preferably an alkyl alcohol such as methanol, ethanol, n-propanol or i-propanol. It is preferred that the good solvent for the C- and S-polymers is such an alkyl alcohol, since the good solvent may be used as it is, for the esterification reaction of the C-polymer.
The obtained ester of the C-polymer may be used in the form of the ester depending upon the application for reuse, or it may be further hydrolyzed and used as a carboxylic acid.
The process for recovering the fluoropolymers of the present invention is suitable for recovering the C-polymer and the S-polymer from an ion exchange membrane for electrolysis of sodium chloride, containing the C-polymer and the S-polymer.
The C-polymer for an ion exchange membrane for electrolysis of sodium chloride may be an ion exchange membrane made of a copolymer of tetrafluoroethylene with perfluorovinyl ether having a carboxylic acid group and having an ion exchange capacity of from 0.8 to 1.9 meq/g dry resin. The above copolymer is preferably one obtained by hydrolyzing a precursor which is a copolymer of tetrafluoroethylene with perfluorovinyl ether having a carboxylate group. Here, the perfluorovinyl ether having a carboxylate group is preferably one represented by the formula CF2xe2x95x90CFxe2x80x94(OCF2CFX)pxe2x80x94(O)qxe2x80x94(CF2)rxe2x80x94CO2CH3, wherein p=0 to 3, q is 0 or 1, provided p+qxe2x89xa71, r is 0 to 12, and X is xe2x80x94F or xe2x80x94CF3. Particularly preferred are the following:
CF2xe2x95x90CFOCF2CF2CO2CH3,
CF2xe2x95x90CFOCF2CF2CF2CO2CH3,
CF2xe2x95x90CFOCF2CF (CF3)OCF2CF2CO2CH3.
Further, as the precursor, a three component type copolymer is also preferred which is obtained by polymerizing the following perfluorovinyl ether together with tetrafluoroethylene and perfluorovinyl ether having a carboxylate group.
CF2xe2x95x90CFOCF2CF2CF3,
CF2xe2x95x90CFOCF2CF(CF3)OCF2CF2CF3.
Further, the S-polymer for the above ion exchange membrane for electrolysis of sodium chloride may be an ion exchange membrane made of a copolymer of tetrafluoroethylene with perfluorovinyl ether having a sulfonic acid group, and having an ion exchange capacity of from 0.8 to 1.3 meq/g dry resin. The above copolymer is preferably one obtained by hydrolyzing a precursor which is a copolymer of tetrafluoroethylene with perfluorovinyl ether having a sulfonyl fluoride group. Here, the perfluorovinyl ether having a sulfonyl fluoride group is preferably one represented by the formula CF2xe2x95x90CFxe2x80x94(OCF2CFZ)sxe2x80x94(O)txe2x80x94(CF2)uxe2x80x94SO2F wherein s=0 to 3, t is 0 or 1, provided s+txe2x89xa7=1, u is 0 to 12, and Z is xe2x80x94F or xe2x80x94CF3. Particularly preferred are the following.
CF2xe2x95x90CFOCF2CF2SO2F,
CF2xe2x95x90CFOCF2CF(CF3)OCF2CF2SO2F.
Further, in the present invention, in a case where the ion exchange membrane is laminated with a reinforcing material such as a woven fabric made of PTFE, it is preferred to remove the reinforcing material at the time of removing the inorganic particles, or by dissolving the ion exchange membrane in a good solvent, followed by filtration.
According to the present invention, since the C-polymer and the S-polymer can be recovered in high purity, respectively, it is possible to reuse them without purification after the recovery. However, if a higher purity is required depending upon the particular purpose, purification is carried out. As a purification method, preferred is a method wherein the recovered C-polymer or S-polymer is subjected to heat treatment in an alcohol such as methanol in the presence of sulfuric acid, and the resulting ester (solid) of the C-polymer is separated and removed.
The C-polymer or its esterified compound recovered by the present invention, can be reused as a material for an ion exchange membrane for electrolysis of sodium chloride. The S-polymer is useful as a membrane material for a fuel cell, as a material for an ion exchange membrane for electrolysis of sodium chloride or as a material for fluororesin fibers.
In the recovery process of the present invention, a specific example of permitting an ion exchange membrane to swell thereby to remove inorganic particles, will be as follows.
An ion exchange membrane having a woven fabric of PTFE laminated and inorganic particles deposited on the surface, is cut and then treated with an acid solution e.g. a mixed solution of a 10 mass% hydrochloric acid aqueous solution and ethanol (10 mass% hydrochloric acid aqueous solution/ethanol=90%/10%). Then, filtration is carried out by means of a sieve, and the obtained filtered product is washed with a washing liquid such as water to remove the inorganic particles (the filtered product is the polymer components, and the inorganic particles are dispersed in the filtrate). The filtered product is dissolved in methanol as a good solvent, whereupon a mixed solution of the C-polymer and the S-polymer is withdrawn, and the woven fabric of PTFE is removed by filtration. Then, the mixed solution of the C-polymer and the S-polymer is heated to let the methyl ester of the C-polymer precipitate, whereupon the methyl ester of the C-polymer as solid and the S-polymer as liquid are separated by solid-liquid separation.
In the process for recovering fluoropolymers of the present invention, a specific example of contacting an ion exchange membrane with a good solvent thereby to remove inorganic particles, will be as follows.
An ion exchange membrane having a woven fabric of PTFE laminated and inorganic particles deposited on the surface, is cut and treated with an acid solution e.g. a mixed solution of a 10 mass% hydrochloric acid aqueous solution and ethanol (volume ratio: 10 mass% hydrochloric acid aqueous solution/ethanol=90%/10%), to convert all of the C-polymer and the S-polymer to acid-forms. Then, they are contacted with methanol as a good solvent, whereupon a mixed solution of the C-polymer and the S-polymer is withdrawn, and at the same time, the inorganic particles and the woven fabric of PTFE are removed by filtration. The mixed solution of the C-polymer and the S-polymer, is heated to let a methyl ester of the C-polymer precipitate, and the methyl ester of the C-polymer as solid, and the S-polymer as liquid, are separated by solid-liquid separation.